1. In a water to air heat exchanger, it is important to place fins on the air side because a. The air side heat transfer coefficient is so much lower than on the water side. b. It reduces drag in high speed flows c. The air side heat transfer coefficient is higher than that of the water side d. It affects good mixing of the gas and this promotes good heat transfer 2. In a heat exchanger, when is the heat transfer maximum? a. T(cold inlet) = T(hot inlet) b. T(cold inlet) = T(cold outlet) c. T(cold outlet) = T(hot outlet) d. The equipment is new 3. Which of the following a measure of the performance of an evaporator? a. Heat transfer rate b. Steam economy c. Heat transfer coefficient d. Heating surface area 4. Which of the following is most likely the effect of lowering the feed temperature in an evaporator? a. Decreased heating area requirement b. Reduced steam consumption c. Reduced steam economy d. Increased capacity 5. A greater wall thickness of the packing material (i.e. Raschig Rings) will result a / an a. Decreased superficial velocity b. Greater free space c. Increased surface area d. Increased pressure drop 6. Which of the following is TRUE as regards to gas absorption? a. The operating line is always straight b. The operating line is below the equilibrium line c. The operating line represents a heat balance d. For efficient mass transfer, the resistance in the liquid film should be low.

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EXPERT ANSWER

1)a) option is the right answer.

Fins are used to increase heat transfer where heat transfer coefficient is low by increasing effective heat transfer area. This statement supports first option is the best choice.

2) d) option is correct

= UAAT

heat transfer rate Q is directly proportional to heat transfer coefficient(U), effective area(A) and temperature difference of cold and hot inlet. So, first three option does not convey anything about temperature difference of cold and hot inlet. Thus, we can not conlude anything from these statements about heat transfer. But fourth option says that equipment is new. Actually, old equipment’s heat transfer coefficient decreases due to fouling. Hence, new equipment has higher U. Hence, higher Q.

3) b) is the answer.

Steam economy is basically measurement of ratio of amount of water vaporized and amount of steam used. This parameter is defined only for judging the performance of evaportor. This eliminates rest options.

4) d) is the answer.

If feed temperature is low, we require more heat for heating the feed. Evaporator capacity is basically the amount of steam used to heat the feed for evaporation in 1hr. This eliminates all other options.